In order to provide efficient data transmission in Long Term Evolution (LTE) system, downlink control channel known as physical downlink control channel (PDCCH) is provided. A PDCCH carries a message known as downlink control information (DCI), which includes transmission resource assignments and other control information for a user equipment (UE) or group of UEs.
In LTE Rel-8/9/10, downlink control channels comprise physical control format indicator channel (PCFICH), physical downlink control channel (PDCCH) and physical HARQ indicator channel (PHICH). Current downlink control signaling is based on common reference signals (CRS) which are not precoded and are broadcast all over the cell. When multiple antennas are in use, transmit diversity is used by means of space-frequency bock code (SFBC) in case of 2 transmit antennas and space-frequency block code-frequency switched transmit diversity (SFBC-FSTD) in case of 4 transmit antennas. The mapping of the control channels to resource elements (REs) is fixed and based on the cell identifier (ID). Downlink control channels are mapped to REs over the full frequency band of the system in a so-called “control channel region” the width of which in number of orthogonal frequency division multiplexing (OFDIM) symbols from the start of the LTE subframe is indicated by PCFICH. The PHICH channel carries acknowledgement/negative-acknowledgement (ACK/NACK) indications in support of uplink (UL) synchronous hybrid automatic repeat request (HARQ) operation.
LTE Rel-8/9/10 uses a hybrid HARQ scheme for error correction in addition to forward error correction (FEC) with channel coding. Uplink transmission operates along synchronous HARQ, meaning that occurrence for retransmission for a given HARQ process happen at predefined time instances. In response to a correctly and incorrectly, respectively, received uplink transmission from the UE, the evolved NodeB (eNB) sends an ACK and a NACK, respectively, to the UE in the form of a HARQ indicator codeword transmitted over the PHICH. HARQ indicator for a given HARQ process and codeword is either set to 0 (NACK) or 1 (ACK). It is then channel-coded with repetition coding to a set of three bits (that is “000” or “111”) to form a HARQ indicator codeword which undergoes BPSK modulation, scrambling, layer mapping, precoding and resource mapping as shown in FIG. 1.
Such modulation and scrambling is implemented as follows:
      d    ⁡          (      i      )        =            w      ⁡              (                              i            ·            mod                    ⁢                                          ⁢                      N            SF            PHICH                          )              ×          (              1        -                  2          ⁢                      c            ⁡                          (              i              )                                          )        ×          z      ⁡              (                  ⌊                      i                          N              SF              PHICH                                ⌋                )            withw(i·mod NSFPHICH) as the orthogonal sequence symbol with index nPHICHseq (1−2c(i)) as the cell specific scrambling sequence symbol, and
  z  ⁡      (          ⌊              i                  N          SF          PHICH                    ⌋        )  as the modulated HARQ indicator symbol.
A further illustration of the modulation and scrambling is shown in FIG. 2.
Multiple PHICHs are mapped to the same set of REs via the use of orthogonal spreading sequences in order to constitute a PHICH group. Elements of PHICH sequences are summed to create the sequence of the corresponding PHICH groups, which is shown in FIG. 3. Spreading sequences are either all real- or all complex-valued orthogonal sequences of length-4 for normal cyclic prefix (CP) and length-2 for extended CP. Cell-specific scrambling is applied in order to mitigate inter-cell interference. A PHICH resource occupies twelve REs for normal CP and six REs for extended CP. PHICH REs are aligned and mapped to resource element groups (REG) which consist of four consecutive REs in frequency domain, each containing one complex scrambled symbol.
Complex symbols are mapped to one, two or four layers which are then correspondingly precoded over antenna ports with respectively single antenna port transmission, two antenna port SFBC transmit diversity or four antenna port SFBC-FSTD transmit diversity. The number of OFDM symbols used for PHICH mapping is configurable by the PHICH duration, which is either normal (OFDM symbol 0 in the subframe) or extended (OFDM symbols 0, 1, 2 in general except for in time division duplex (TDD), where subframes 1 and 6 are used, or in multimedia broadcast single frequency network (MBSFN) subframes, where OFDM symbols 0 and 1 are used). Mapping of PHICH to OFDM symbols is illustrated in FIG. 4.
A PHICH resource is identified by its group index and sequence index, which are linked to the uplink demodulation reference signal (DMRS) cyclic shift.
The PHICH resource is identified by the index pair (nPHICHgroup, nPHICHseq) where nPHICHgroup is the PHICH group number and nPHICHseq is the orthogonal sequence index within the group as defined by:nPHICHgroup=(IPRB,RA+nDMRS)mod NPHICHgroup+IPHICHNPHICHgroup nPHICHseq=(└IPRB—RA/NPHICHgroup┘+nDMRS)mod 2NSFPHICH wherenDMRS is determined by use of the following mapping table the information known from the “cyclic shift for DM RS”-field in the most recent PDCCH with uplink DCI format for the transport block(s) associated with the corresponding PUSCH transmission:
“cyclic shift for DM RS”-field in thePDCCH with the uplink DCI formatnDMRS00000011010201131004101511061117nDMRS shall be set to zero                if there is no PDCCH with uplink DCI format for the same transport block, and        if the initial PUSCH for the same transport block is semi-persistently scheduled, or        if the initial PUSCH for the same transport block is scheduled by the random access response grant.NSFPHICH is the spreading factor size used for PHICH modulation,        
      I    PRB_RA    =      {                                                                                        for            ⁢                                                  ⁢            the            ⁢                                                  ⁢            first            ⁢                                                  ⁢            TB            ⁢                                                  ⁢            of            ⁢                                                  ⁢            a            ⁢                                                  ⁢            PUSCH            ⁢                                                  ⁢            with            ⁢                                                  ⁢            associated            ⁢                                                  ⁢            PDCCH            ⁢                                                  ⁢            or            ⁢                                                  ⁢            for            ⁢                                                  ⁢            the            ⁢                                                  ⁢            case            ⁢                                                  ⁢            of                                                                                                            no            ⁢                                                  ⁢            associated            ⁢                                                  ⁢            PDCCH            ⁢                                                  ⁢            when            ⁢                                                  ⁢            the            ⁢                                                  ⁢            number            ⁢                                                  ⁢            of            ⁢                                                  ⁢            negatively            ⁢                                                  ⁢            acknowledged                                                            I            PRB_RA            lowest_index                                                TBs            ⁢                                                  ⁢            is            ⁢                                                  ⁢            not            ⁢                                                  ⁢            equal            ⁢                                                  ⁢            to            ⁢                                                  ⁢            the            ⁢                                                  ⁢            number            ⁢                                                  ⁢            of            ⁢                                                  ⁢            TBs            ⁢                                                  ⁢            indicated            ⁢                                                  ⁢            in            ⁢                                                  ⁢            the            ⁢                                                  ⁢            most            ⁢                                                  ⁢            recent                                                                                                            PDCCH            ⁢                                                  ⁢            associated            ⁢                                                  ⁢            with            ⁢                                                  ⁢            the            ⁢                                                  ⁢            corresponding            ⁢                                                  ⁢            PUSCH                                                                          I              PRB_RA              lowest_index                        +            1                                                for            ⁢                                                  ⁢            a            ⁢                                                  ⁢            second            ⁢                                                  ⁢            TB            ⁢                                                  ⁢            of            ⁢                                                  ⁢            a            ⁢                                                  ⁢            PUSCH            ⁢                                                  ⁢            with            ⁢                                                  ⁢            associated            ⁢                                                  ⁢            PDCCH                              whereIPRB—RAlowest—index is the lowest PRB index in the first slot of the corresponding PUSCH transmission,NPHICHgroup is the number of PHICH groups configured by higher layers, and
      I    PHICH    =      {                            1                                                    for              ⁢                                                          ⁢              TDD              ⁢                                                          ⁢                              UL                /                DL                            ⁢                                                          ⁢              configuration              ⁢                                                          ⁢              0              ⁢                                                          ⁢              with              ⁢                                                          ⁢              PUSCH              ⁢                                                          ⁢              transmission              ⁢                                                          ⁢              in              ⁢                                                          ⁢              subframe              ⁢                                                          ⁢              n                        =                          4              ⁢                                                          ⁢              or              ⁢                                                          ⁢              9                                                            0                          otherwise                    
In LTE frequency division duplex (FDD), the number of PHICH groups is constant in all subframes and is configured via higher layer signaling. In LTE TDD, the number of PHICH groups depends additionally on the subframe number and uplink/downlink TDD configuration.
In continuation of the LTE concept, in a planned release an enhanced physical downlink control channel (E-PDCCH) will resemble the control channel specified in Release 10 for relay nodes, R-PDCCH, in the sense that it will be based on UE-specific reference signals (UE-RS) and be mapped to the PDSCH region of the subframe. E-PDCCH is expected to have support for closed-loop (precoded) spatial multiplexing (at least for rank-1) as well as multi-user (MU) multiple-input multiple-output (MIMO) or coordinated multi-point transmission (CoMP) and/or (in contrast to R-PDCCH) for multiplexing of DCIs to several UEs within one PRB pair. It is noted that the definition of UE-specific RS hints that they are to be used by a single UE, however in principle nothing forbids the eNB to configure multiple UEs with the same RS in which case also UE-specific RS could be in fact shared by multiple UEs.
Currently, 3rd Generation Partnership Project (3GPP) RAN1 is working on the next release of the LTE specifications, i.e. Release 11. Downlink control signaling enhancements were first investigated as part of the study items on downlink MIMO enhancements and CoMP. During development, a new Rel-11 work item on introducing a new downlink control channel was approved. The goal of such new downlink control channel is to specify (an) enhanced physical downlink control channel(s) that is/are able to operate on legacy carriers and on a new carrier type (as planned on carrier aggregation (CA) enhancements for LTE). The enhanced physical downlink control channel(s) shall be able to support increased control channel capacity, to support frequency domain inter-cell interference coordination (ICIC), to achieve improved spatial reuse of control channel resource, to support beamforming and/or diversity, to operate on the new carrier type and in MBSFN subframes, whereby ability to operate in non-MBSFN subframes is also assumed, and to coexist on the same carrier as legacy UEs. It is also desirable for the enhanced physical downlink control channel to be able to be scheduled frequency-selectively, and to be able to mitigate inter-cell interference.